1. Field of the Invention
The present invention relates to cement additives for inhibiting concrete-deterioration and for inhibiting the corrosion of reinforcing steel caused by chloride ions contained in the concrete and the alkali-aggregate reaction caused by alkali metal ions also contained in the concrete. More particularly, the present invention relates to cement additives containing as effective components an inorganic cation exchanger and an inorganic anion exchanger.
2. Description of the Background
Concrete is widely used in modern construction, for structures, buildings and the like because of its excellent properties and low cost achieved by mass production. It is now an indispensable construction material.
Typically, concrete contains a mixture of gravel, pebbles, sand, broken stone, blast-furnace slag or cinders, termed the aggregate, embedded in a matrix of either mortar or cement, usually Portland cement. Reinforced concrete and retro-concrete contain steel in some form.
However, construction and the like of concrete, which were once believed to be extremely strong and semipermanent, have turned out to be subject to deterioration caused by various internal and external factors. The deterioration of concrete constructions is now a serious concern.
Among the causes of deterioration of concrete, salt damage and alkali-aggregate reactions are particularly significant factors causing the deterioration of concrete. "Salt damage" is chloride ion-caused corrosion of the reinforcing steel used in concrete constructions.
Chloride ions are mainly introduced into concrete in the form of its constituent materials, that is, calcium chloridecontaining additives, which are often used in large amounts, and sea sand, which is reluctantly but increasingly used as an aggregate because river and pit sands have become scarce. Besides such internal chloride ions, external chloride ions enter the concrete of construction in the form of, for example, splashes of sea water, or a calcium chloride solution which is sprayed on roads to inhibit freezing or to melt the snow thereon.
Conventional countermeasures for salt damage have merely treated the consequences arising from the fundamental cause, that is, chloride ions. For example such countermeasures include: using of a rust preventative, such as calcium nitrite; coating a construction; and restoring or repairing a construction after deterioration is found. Although it is preferable to eliminate chloride ions from sea sand by, for example, thoroughly washing the sand, complete or even substantial elimination of chloride ions is difficult and usually costly according to the conventional art.
"Alkali-aggregate reaction" means reactions between alkali metal ions contained in cement and reactive minerals contained in an aggregate. The products of the alkali-aggregate reaction expand in the concrete causing cracks, warping, separation, etc. The concrete thus deteriorates.
To prevent or avoid the alkali-aggregate reaction, the following measures are conventionally taken:
1. Avoiding a highly reactive aggregate. PA1 2. Using a cement having a low alkali content. PA1 3. Using an additive having a pozzolan effect, such as blast furnace slag, fly ash or silica fume. PA1 4. Adding salts of aluminium, zinc, etc. PA1 5. Adding silicates such as zeolites, clay minerals, etc. PA1 6. Waterproofing for blocking moisture. PA1 (a) Japanese Patent Application Laid-open Nos. 62-21739, 63-117939 and 63-117940 propose a method in which 1-20 wt % of a powder or granule prepared by baking zeolite-containing green tuff or clay minerals and removing hygroscopic water and crystal water therefrom is added to a concrete. PA1 (b) Japanese Patent Application Laid-open No. 63-274644 proposes a method in which a natural zeolite, such as clinoptirolite, loaded with calcium ions or lithium ions is added to a concrete. PA1 (1) Originally adding it alone or together with other additives to a cement composition so as to inhibit the deterioration of concrete. PA1 (2) Charging it alone or together with other additives into a concrete body whose reinforcing steel is expected to be corroded by salt damage, so as to prevent or inhibit the corrosion thereof. PA1 (3) Applying it alone or together with other additives as a crack or surface repairing agent to a concrete body whose surface has peeled or been damaged by salt damage.
In connection with measure 5 above, several methods have been proposed, for example:
Most zeolites, either natural or synthetic, are typical inorganic cation exchangers. They also have selective adsorbability. Therefore, zeolites are widely used as, for example, a detergent builder, a catalyst, an additive for synthetic resin.
Because zeolites have large cation-exchange capacity, they naturally adsorb alkali metal ions. However, use of zeolites as adsorbents of alkali metal ions has not been fully considered or studied for inhibiting concrete deterioration.
It is known that hydrocalumite is a layer crystalline compound in the form of Ca--Al double hydroxides, which is one type of bivalent-trivalent metal double hydroxides and has anion-exchange capacity. Hydrotalcite is the Mg--Adouble hydroxides, that is, another type of bivalent-trivalent metal double hydroxides. Hydrotalcite is used as a resin additive.
The prevention or delay and repair of the deterioration of concrete construction caused by various factors is now an urgent issue. However, a fundamental and practical measure has not been developed. For example, the above-mentioned removal or confinement of chloride ions in concrete is technically feasible but economically impracticable in most cases.